In the cytological observations of the hypophysis in mice with leukemia or mammary cancer, the author made the classification and calculation of epithelical cells of mouse anterior hypophysis. The results of this experiment were as follows: 1. In the pituitary of mice with leukemia, acidophilic cells decreased in number, while the basophilic particularly chromophobic ones increased in number. 2. In the pituitary of mice with mammary cancer, acidophilic and basophilic cells decreased in number, while the chromophobic ones increased in number. 3. From these findings it is assumed that the hypophyseal function of mice with leukemia or mammary cancer is hormone-unbalanced.